New IPCC Report (Part 3)

A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions.

(Fig 12.43) Results from 1,000 year simulations from EMICs on the 4 RCPs up to the year 2300, followed by constant composition until 3000.

The conclusions about irreversibility of climate change are greatly strengthened from the previous assessment report, as recent research has explored this in much more detail. The problem is that a significant fraction of our greenhouse gas emissions stay in the atmosphere for thousands of years, so even if we stop emitting them altogether, they hang around, contributing to more warming. In simple terms, whatever peak temperature we reach, we’re stuck at for millennia, unless we can figure out a way to artificially remove massive amounts of CO2 from the atmosphere.

The graph is the result of an experiment that runs (simplified) models for a thousand years into the future. The major climate models are generally too computational expensive to be run for such a long simulation, so these experiments use simpler models, so-called EMICS (Earth system Models of Intermediate Complexity).

The four curves in this figure correspond to four “Representative Concentration Pathways”, which map out four ways in which the composition of the atmosphere is likely to change in the future. These four RCPs were picked to capture four possible futures: two in which there is little to no coordinated action on reducing global emissions (worst case—RCP8.5 and best case—RCP6) and two on which there is serious global action on climate change (worst case—RCP4.5 and best case—RCP 2.6). A simple way to think about them is as follows. RCP8.5 represents ‘business as usual’—strong economic development for the rest of this century, driven primarily by dependence on fossil fuels. RCP6 represents a world with no global coordinated climate policy, but where lots of localized clean energy initiatives do manage to stabilize emissions by the latter half of the century. RCP4.5 represents a world that implements strong limits on fossil fuel emissions, such that greenhouse gas emissions peak by mid-century and then start to fall. RCP2.6 is a world in which emissions peak in the next few years, and then fall dramatically, so that the world becomes carbon neutral by about mid-century.

Note that in RCP2.6 the temperature does fall, after reaching a peak just below 2°C of warming over pre-industrial levels. That’s because RCP2.6 is a scenario in which concentrations of greenhouse gases in the atmosphere start to fall before the end of the century. This is only possible if we reduce global emissions so fast that we achieve carbon neutrality soon after mid-century, and then go carbon negative. By carbon negative, I mean that globally, each year, we remove more CO2 from the atmosphere than we add. Whether this is possible is an interesting question. But even if it is, the model results show there is no time within the next thousand years when it is anywhere near as cool as it is today.

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13 Responses to New IPCC Report (Part 3)

You don’t identify what is on this graph in the level of detail that you did in the graph on part 2, and taken together that might be misleading. Panel b, has shaded areas that look like what you called “uncertaintly bars” on the part 2 figure. But these are not uncertainty bars, but just the range of model results. No attempt is made by the IPCC authors to quantify the uncertainty introduced by the wealth of diagnostic issues they previously reviewed and discussed. Without mentioning this, someone moving from the previous graph to this one might misinterpret the graph.

I don’t really buy the argument that this might mislead. We’re talking about projections of future change. The only possible error bar that can be offered on such projections is the range of model results for different models, because other aspects of uncertainly (e.g. structural uncertainty in the models) cannot be reduced to a single value. But it’s not true to claim that these uncertainties remain unexamined by the IPCC (or more specifically, by the model inter-comparison projects that provide inputs to the IPCC). The full set of model experiments in the CMIP5 contain many sets that are specifically designed to probe the sensitivity of models to different assumptions. The results from this analysis can’t provide a simple error bar on model projections, but they show up in the IPCC report in terms of expert judgement of confidence in the projections.

So, the graph represents our best understanding of the long term temperature changes as a result of elevated GHG levels. The models might be wrong in either direction in terms of the magnitude of the temperature anomalies. But the overall shape of the curve is almost certainly correct, and that’s the point I want to get across in this post.

I assume the scenarios don’t allow the CO2 to fall after 2200 is order to find out what the equilibrium climate sensitivity for that level of CO2 forcing is. As RCP2.6 demonstrates before the year 2200, this statement is not quite as true as the graphs seem to indicate:

“The problem is that a significant fraction of our greenhouse gas emissions stay in the atmosphere for thousands of years, so even if we stop emitting them altogether, they hang around, contributing to more warming.”

The CO2 levels get frozen after 2200 for RCP2.6 as well.

What doesn’t show up in this graph or the author’s assessments is that even just the correlated errors are several times the size of the energy imbalance that is being attributed and projected. That is why they don’t put uncertainty bars on the model projections.

Clearly, it would be very useful to “figure out a way to artificially remove massive amounts of CO2 from the atmosphere”, even if we *also* drastically limit CO2 addition in the short term. Is there much funding for work on that?

Yes, there’s lots of work going on for this. Google for the phrase “Direct Air capture of CO2” for some examples. However, I doubt this technology will do much more than buy us a little time. The problem is that as long as our CO2 emissions keep rising, our deployment of this technology would also have to keep growing to keep up. It’s a kind of arms race that will be hard to win. Much better to tackle the underlying cause.

The other problem is economics. We use fossil fuels because they are the cheapest source of energy, although the prices are rising steadily as we use up the easier sources. Once you add in the growing cost of the extra infrastructure needed for carbon capture and storage, fossil fuels rapidly become uncompetitive in price. Which means that either we’ll continue to use them without CCS, and ignore the consequences, or we’ll switch to clean energy. It’s hard to imagine a future in which fossil fuels with CCS become the dominant energy source.

In the longer term, some future generation might invent a technology to undo what we’re currently doing to the atmosphere. But I fear they’ll be too busy just trying to survive on a devastated planet.

Suppose we can limit CH4 addition to atmosphere in different ways (at different costs/timescales) as CO2 addition. Has it been studied which one might be more cost-effective to do? Do the RCP scenarios assume some sort of “analogous limiting” for CH4 as for CO2, or no change from business as usual in CH4? (I realize that limiting CH4 alone probably wouldn’t help with ocean acidification.)

I don’t know how to slow methane emission from melting permafrost, which may not be included in this chart. But it’s clear that cattle play a big role in methane emissions, so one question is: how much does it cost to persuade people to eat less beef?

The greenhouse gas emissions produced by raising livestock greatly exceed their carbon footprint due to CO2 emissions, because cattle burp out a lot of methane, and there are also large N2O emissions—both these greenhouse gases have a much higher global warming potential than CO2, at least in the short run. See:

I don’t know much about relative costs, but I have seen interesting research on the relative impacts. The studies show that reducing CH4 emissions will affect the peak warming, while reducing CO2 emissions will affect the long term temperature change. In the longer term, the distinction doesn’t matter much, because CH4 breaks down in the atmosphere into CO2 and water.

You’re misreading the graph. Every minor checkmark on the x axis is fifty years. So “today’s value” is pretty much zero on the temperature graph. And every minor checkmark on the y axis is 0.5degC. RCP2.6 peaks at about 1C warmer than today, and ends up about half a degree warmer. Both these temperatures are warmer than any time in the last million years.

Thanks Steve. Yup I understood the axis scaling. OK I marked up my flat screen more carefully and got 0.6 C vs 0.3, for a difference of 0.3 deg C. Which doesn’t seem to justify “[not] anywhere near” given how much this stat has fluctuated historically on a timescale of centuries.

The ‘last million years’ observation, on the other hand, really got my attention.

How To Write Math Here:

You need the word 'latex' right after the first dollar sign, and it needs a space after it. Double dollar signs don't work, and other limitations apply, some described here. You can't preview comments here, but I'm happy to fix errors.